In forthcoming evolutions of cellular radio communication system standards, such as Long Term Evolution (LTE) and High-Speed Packet Access (HSPA), the maximum data rate will surely be higher than in previous systems. Higher data rates typically require larger system channel bandwidths. For an IMT advanced system (i.e., a “fourth generation” (4G) mobile communication system), bandwidths of 100 megahertz (MHz) and larger are being considered.
LTE and HSPA are sometimes called “third generation” communication systems and are currently being standardized by the Third Generation Partnership Project (3GPP). The LTE specifications can be seen as an evolution of the current wideband code division multiple access (WCDMA) specifications. An IMT advanced communication system uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, HSPA, or other communication system for IMS multimedia telephony (IMT). The 3GPP promulgates the LTE, HSPA, WCDMA, and IMT specifications, and specifications that standardize other kinds of cellular wireless communication systems.
An LTE system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs). An LTE system has channel bandwidths ranging from 1.25 MHz 30 to 20 MHz, and supports data rates up to 100 megabits per second (Mb/s) on the largest-bandwidth channels. One type of physical channel defined for the LTE downlink is the physical downlink shared channel (PDSCH), which conveys information from higher layers in the LTE protocol stack and is mapped to one or more specific transport channels. The PDSCH and other LTE channels are described in 3GPP Technical Specification (TS) 36.211 V8.1.0, Physical Channels and Modulation (Release 8) (November 2007), among other specifications.
In an OFDMA communication system like LTE, the data stream to be transmitted is portioned among a number of narrowband subcarriers that are transmitted in parallel. In general, a resource block devoted to a particular UE is a particular number of particular subcarriers used for a particular period of time. A resource block is made up of resource elements, each of which is a particular subcarrier used for a smaller period of time. Different groups of subcarriers can be used at different times for different users. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a UE to demodulate each subcarrier. Like many modern communication systems, DL transmissions in an LTE system are organized into frames of 10 milliseconds (ms) duration, and each frame typically includes twenty successive time slots. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
FIG. 1 depicts a typical cellular communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate base station(s) (BSs), which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or Node B in 3G vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the Node Bs 22, 24, 26, and other functionalities can be moved to other nodes in the network. It will also be understood that a base station can use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas can send respective, different pilot signals.
Fast and efficient cell search and received signal power measurements are important for a UE to get and stay connected to a suitable cell, which can be called a “serving cell”, and to be handed over from one serving cell to another. In current LTE specifications, handover decisions are based on measurements of reference signal received power (RSRP), which can be defined as the average UE-received signal power of reference signals or symbols (RS) transmitted by a Node B. A UE measures RSRP on its serving cell as well as on neighboring cells that the UE has detected as a result of a specified cell search procedure.
The RS, or pilots, are transmitted from each Node B at known frequencies and time instants, and are used by UEs for synchronization and other purposes besides handover. Such reference signals and symbols are described for example in Section 7.1.1.2.2 of 3GPP Technical Report (TR) 25.814 V7.0.0, Physical Layer Aspects for Evolved Universal Terrestrial Radio Access (UTRA) (Release 7), June 2006, and Sections 6.10 and 6.11 of 3GPP TS 36.211 cited above. RS are transmitted from each of possibly 1, 2, or 4 transmit antennas of a Node B on particular resource elements (REs) that can be conveniently represented on a frequency-vs.-time plane as depicted in FIG. 2. It will be understood that the arrangement of FIG. 2 is just an example and that other arrangements can be used.
FIG. 2 shows two successive time slots, indicated by the vertical solid lines, which can be called a sub-frame in an LTE system. The frequency range depicted in FIG. 2 includes about twenty-six subcarriers, only nine of which are explicitly indicated. RS transmitted by a first transmit (TX) antenna of a Node B are denoted R and by a possible second TX antenna in the node are denoted by S. In FIG. 2, RS are depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 3 or 4 (depending on whether the symbols have long or short cyclic prefixes) in every slot. Also in FIG. 2, the RSs in symbols 3 or 4 are offset by three subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
Besides reference signals, synchronization signals are needed during cell search. LTE uses a hierarchical cell search scheme similar to WCDMA in which synchronization acquisition and cell group identifier are obtained from different synchronization channel (SCH) signals. Thus, a primary synchronization channel (P-SCH) signal and a secondary synchronization channel (S-SCH) signal are defined with a pre-defined structure in Section 6.11 of 3GPP TS 36.211. For example, P-SCH and S-SCH signals can be transmitted on particular subcarriers in particular time slots. Primary and secondary synchronization signals are described in U.S. patent application Ser. No. 12/024,765 filed on Feb. 1, 2008, by R. Baldemair et al. for “Improved Synchronization for Chirp Sequences”.
Problems can arise in such communication systems because the radio spectrum is a limited resource that must be shared by many systems and operators. For example, it is difficult to find unused continuous blocks of radio frequency (RF) spectrum that are at least 100-MHz wide. One way to solve such problems is to aggregate contiguous and non-contiguous blocks of RF spectrum and thereby—from a baseband point of view—obtain a large enough system RF bandwidth.
FIG. 3 depicts such RF spectrum aggregation, showing two non-contiguous blocks of 20 MHz and one block of 10 MHz, which is contiguous with one of the 20-MHz blocks, aggregated for a total RF bandwidth of 50 MHz. As seen in FIG. 3, the aggregated blocks can be contiguous or non-contiguous, and the artisan will understand that the blocks shown are compliant with the LTE (3GPP Release 8, or Rel-8) specifications.
One benefit of RF aggregation is that it is possible to obtain system RF bandwidths that are large enough to support data rates of one gigabit per second (Gb/s) and even higher, which is a throughput requirement for 4 G communication systems like the IMT-advanced system. Furthermore, RF aggregation makes it possible to adapt the aggregated blocks of RF spectrum to a current communication situation and geographical position, thus giving a communication system desirable flexibility.
The simple RF aggregation described above can even be modified by introducing multi-carrier aggregation, which is to say, aggregation of segments of RF spectrum that are available on different radio carrier signals. Such carrier signals would be carriers in the same cell, for example, an LTE cell, and such a cell can be said to have a “multi-component carrier” or to be a “multi-carrier cell”. A “multi-carrier” LTE UE would simultaneously receive multiple LTE carrier signals that have different frequencies and different bandwidths.
Current cell search techniques in LTE and other communication systems can handle only single-carrier cells, i.e., systems where each cell identity (ID) is associated with a continuous segment of RF bandwidth. After a UE has detected a cell ID on a certain carrier frequency, the UE has, by definition in the current techniques, determined a cell and its cell ID. Thus, if a cell is a multi-carrier cell and a UE has detected a cell ID on a carrier frequency, current cell search techniques say nothing about how to detect a cell ID or cell IDs belonging to other carriers having other frequencies in the same multi-carrier cell.
One solution to this short-coming is to have the current serving cell inform the UE about suitable carrier frequencies and cell IDs on neighboring cells, which is to say that the UE can receive a list of neighbor cells. Nevertheless, that solution is undesirable, as it was in previous cellular communication systems, because it requires extensive cell planning and site coordination, which are typically expensive tasks, and uses system resources for transmitting the neighbor-cell lists.
Therefore, there is a need for methods and apparatus describing how to detect cells and do measurements on cells in a multi-component carrier cellular system without the need for neighbor-cell lists.